Acid-Base Balance — OR & ICU Integration

Three-Domain Architecture

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Cognitive

Core Content

Henderson-Hasselbalch physiology; Traditional classification (respiratory/metabolic acidosis/alkalosis); Stewart physicochemical approach (SID, Atot, SIG); Metabolic acidosis — anion gap, delta ratio subgroups G1/G2/G3 (G2 HR 3.72 for mortality); Metabolic alkalosis — chloride-responsive vs resistant (OR 4.87 mortality); Mixed disorder recognition; Fluid therapy acid-base effects; Delta AG/ΔHCO₃⁻ for mixed disorder detection.

Provides the "why" for acid-base management — understanding physiology, multiple methodological approaches, and prognostic implications guides systematic, individualized care.

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Psychomotor

Core Content

Systematic ABG interpretation (8-step approach); Anion gap calculation with albumin correction; Delta gap calculation (ΔAG/ΔHCO₃⁻) and G1/G2/G3 classification; Winter's formula for metabolic acidosis; Urine chloride for metabolic alkalosis; Stewart variable calculation (SIDa, SIDe, SIG); Compensation formula application; Fluid selection based on acid-base goals.

Translates knowledge into action — the right calculations, the right classification, the right therapeutic response for acid-base disorders.

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Affective

Core Content

Clinical judgment (integrating acid-base data with patient presentation); Diagnostic humility (9% of acidosis missed by traditional methods); Speaking up about critical values (G2 mortality HR 3.72); Vigilance for metabolic alkalosis (OR 4.87 mortality); Patient advocacy (preventing iatrogenic acid-base disorders); Debriefing after acid-base crises.

Transforms technical competence into safe, team-based, patient-centered acid-base management.

Evidence-Based Key Insights

01 · Henderson-Hasselbalch

Foundational Equation

pH = 6.1 + log([HCO₃⁻] / (0.03 × PaCO₂))

Acid-base balance is essential for enzyme function, cellular signaling, and ATP synthesis.

02 · Stewart Approach

Physicochemical Model

Three independent determinants: SID (strong ion difference), Atot (total weak acids), and PCO₂. Identifies abnormalities missed by traditional methods in 9% of cases.

03 · Delta Gap

ΔAG/ΔHCO₃⁻ Prognostic Groups

G1 (<1.0): MA with hyperchloremia
G2 (1.0–1.6): Pure high AG acidosis HR 3.72 mortality
G3 (>1.6): MA with alkalosis

04 · Metabolic Alkalosis

Mortality Risk in ICU

OR 4.87 for in-hospital mortality in critically ill patients. Related to hypoalbuminemia and hypokalemia.

05 · Anion Gap Correction

Albumin-Corrected AG

Corrected AG = AG + 2.5 × (4.5 – albumin)

Hypoalbuminemia masks high anion gap acidosis. Correction is mandatory in critically ill patients.

06 · Winter's Formula

Expected Respiratory Compensation

Expected PaCO₂ = (1.5 × HCO₃⁻) + 8 ± 2

Values outside this range indicate mixed metabolic and respiratory disorders.

07 · Fluid-Induced Acidosis

Saline vs Balanced Crystalloids

0.9% saline (SID = 0) causes hyperchloremic metabolic acidosis. Balanced crystalloids (SID ~28–50) minimize acid-base disturbance.

08 · Diagnostic Accuracy

Traditional vs Stewart

13 of 149 patients (9%) with metabolic acidosis identified by Stewart were not detected by traditional methods. Using multiple approaches improves diagnostic accuracy.

09 · Strong Ion Gap

SIG Calculation

SIG = SIDa – SIDe (Normal: 0 ± 2)

Elevated SIG indicates unmeasured anions: ketones, toxins, and uremic acids.

10 · Compensation

Mixed Disorder Detection

Inadequate or excessive compensation indicates mixed disorders. A systematic approach prevents missing complex acid-base pictures.

This module joins a comprehensive anaesthesiology knowledge base now covering 36 complete modules following the same three-domain architecture — spanning the full perioperative continuum and all subspecialty areas.

Module 36 of 36